The invention relates to a piston for all internal combustion engine comprising:
a piston head provided with at least one piston ring groove, PA1 a piston body having a piston pin bore, the piston body having a largest diameter region, and PA1 a compression height of less than 40% of the piston diameter.
A piston of a combustion engine is connected with a crankshaft in the known manner via a connecting rod and guided for a reciprocating movement in a cylinder. During this movement of the piston, before a change in direction thereof, the piston reaches a top or bottom dead centre in which the side of the piston which lies against the cylinder wall changes due to the change in the direction of movement of the piston. The tilted movement conducted in this case by the piston takes place about the axis of the piston pin by which the piston is connected with the connecting rod.
Due to the high revolutions of the engine, the described tilting movement of the piston leads to a relatively strong striking of the piston against the respective areas of the cylinder wall, which in turn results in a tilting or chattering noise actually caused by the alternation of the side against which the piston strikes. The greater the play is between the piston and the cylinder wall, the greater the noise. In order to achieve a motion of the piston which generates as little noise as possible, it is therefore important that the play at assembly between the piston and the cylinder wall is made as small as possible; in Otto-cycle engines, for example, the standard play at assembly at the narrowest point, i.e. in the region in which the piston has its largest diameter, is approximately 25 micrometers. However, the selection of the play at assembly and the fitting-in of a piston have proved to be difficult on account of the thermal loads on the piston. As the combustion engine should run quietly in all operating conditions, i.e. for a cold motor as well as in a greatly heated condition, and that in all of these operational conditions, a jamming or seizing of the piston must also be prevented, the expansion of the piston resulting from the heating up thereof must be taken into account. It is therefore usual to provide the piston head, which is subjected to the highest temperature loads, with a somewhat smaller diameter, and to form the part of the piston at which it has its greatest diameter and with which it lies against the cylindrical wall at the piston skirt. On account of the distance of the region of greatest diameter from the top land of the piston, the operational temperatures of the engine have a smaller effect on this greatest diameter region.
The pistons usually applied up to now have a diameter--total length relationship of approximately 1 to 1, i.e. they are formed approximately "quadratically"; pistons of smaller diameter are sometimes provided with a somewhat greater total length. As is known, the length of a piston is divided into the compression height and the lower body length, whereby the compression height is the distance between the centre of the piston pin bore and the edge of the piston head, while the lower body length represents the length from the centre of the piston pin bore to the end of the body. Usually, the compression height amounts to approximately 60% of the piston diameter.
In the development of ever lighter engines which are also to be operated at ever increasing revolutions, it has been attempted to reduce the moving mass in the engine as much as possible. Here in particular, the weight of the piston plays a decisive role. For the reduction in weight of pistons, apart from the selection of material, the possibility exists to reduce the height of the solid piston head, i.e. to diminish the compression height. This has occured up to values of less than 40% of the piston diameter. In order to ensure a reliable guidance of the piston, the lower piston body length must be simultaneously increased so that the surface pressure stemming from the lateral forces does not exceed certain given values, for example values at which the oil film would be squeezed away from between the piston and the cylinder. By reducing the compression height, the zone of heat influence arising from the heating up of the piston head inevitably moves nearer to the piston pin bore or the eye of the pin so that the largest diameter region must be displaced toward the lower body area as a necessary consequence, if the diameter, i.e. the narrowest play at assembly, is to be maintained unchanged. On account of this displacement of the region of narrowest play to the lower end of the piston body, an increase in the tilting movement of the piston unavoidably results so that the level of noise greatly increases.
In order to remedy this disadvantage, it has been attempted to embed expansion reducing steel regulating inserts in the aluminium material of the piston. Such a piston is described in EP-Bl-171568. These regulating inserts are to serve to influence the expansion of the piston body in order to be able to place the largest diameter region closer to the axis of the piston pin bore in this manner. However, the manufacturing input increases for this kind of development, and in particular the input for casting the piston is increased substantially; further, considerable difficulties can occur in large volume production. On the whole, this solution has not proved to be completely satisfactory.